One of the primary steps in the fabrication of modern semiconductor devices is the formation of a layer or film over a substrate by chemical reaction of gases. Such a deposition process is referred to generally as chemical vapor deposition (“CVD”). Conventional thermal CVD processes supply reactive precursor gases to a heated chamber in which the substrate is disposed. As the precursors pass over the heated substrate, a pyrolytic reaction causes chemical decomposition and produces the solid layer or film.
There are a number of applications in which one or more precursors are provided as liquids or solids, with vapors from such precursors being transported to the chamber with a carrier gas. One difficulty associated with the use of liquid or solid precursors in such arrangements is the need to determine when the precursor in the bubbler has been fully consumed. This is generally difficult because processing arrangements are such that the source is virtually inaccessible. Typically, the precursor is completely contained within a steel container that is itself disposed within a constant-temperature bath. Conventionally, the bubbler is removed from the processing system and weighed to determine the quantity of precursor remaining. But this is undesirable because it involves breaking leaktight seals and exposing the gas lines to air contamination.
A number of alternative techniques have accordingly been used. These include capacitance probing, in which the steel bubbler itself forms one plate of a capacitor. A rod that is inserted into the center of the bubbler acts as the other plate and the liquid or solid precursor acts as the dielectric. Another technique uses ultrasonic detection, in which an ultrasonic transducer is used to bounce sound off the precursor surface to determine its height within the bubbler. Still another technique measures the pressure head associated with the height of the liquid column contained in the bubbler.
Each of these techniques is generally difficult to use with solid precursors, with the last technique not working at all for solid precursors. In addition, the cost of the instruments needed to perform the measurements for each of the techniques with their associated electronics are generally very high. This acts as a barrier to their use even in applications that use liquid precursors.
Other methods that are sometimes used as a surrogate for measurement of the fill level make a direct determination of the precursor vapor in the carrier gas. For instance, this may be done by measuring the speed of sound in the gas mixture since the speed of sound varies with the relative concentration of certain precursors within certain carrier gases. Such techniques use instrumentation that is quite expensive and, while they offer very good control of gas-mixture compositions, they are not directly useful for determining bubbler fill level.
There is a accordingly a general need in the art for methods and systems for measuring precursor amounts in bubbler sources.